Within the European DVB (Digital Video Broadcasting) system, digital transmission systems have been developed for satellite (DVB-S), for cable (DVB-C) and for terrestrial digital broadcast radio transmission (DVB-T), and appropriate specifications have been worked out for this purpose. As a result of the problematic transmission conditions which occur on the terrestrial radio channel, the OFDM (Orthogonal Frequency Division Multiplexing) transmission method has been specified as the transmission method in the DVB-T specification, which makes it possible to effectively counter the difficult transmission conditions. A further field of application for the OFDM transmission method is formed by high-rate wire-free data transmission networks (WLAN=Wireless Local Area Network), in particular the Standards IEEE 802.11a and IEEE 802.11g, as well as HYPERLAN/2.
The OFDM transmission method is a multicarrier transmission method, in which the datastream is split between a number of parallel (orthogonal) sub-carriers, which are each modulated with a correspondingly low data rate. As is illustrated in FIG. 1, (sub-)carrier frequencies are arranged at equal intervals from one another within a transmission bandwidth K on the frequency scale. The carrier frequencies are located on both sides of and symmetrically with respect to a mid-frequency fc. One OFDM data symbol is formed from the superimposition of all K carrier frequencies. Since the data is transmitted in the form of data blocks of length Tu, which are separated from one another by guard intervals, the time signals in the sub-carriers are multiplied by a square-wave window function. The multiplication of a time signal by a square-wave window function corresponds to convolution of the spectrum with the Si function (si(x)=sin (x)/x, where x=pfTu). The length of a data block, which corresponds to the length of the window, thus determines the interval between the zero points and the Si function, with this frequency interval corresponding to the inverse of the data block length. If the spectrum as shown in FIG. 1 is convolved with the Si function, then this results in the diagram in FIG. 2. FIG. 2 shows the spectrum before the addition of the separate components of each sub-carrier, with four sub-carrier spectra being illustrated by way of example. The maximum of a sub-carrier in the frequency domain is theoretically located precisely at the zero points of all the other sub-carrier spectra. The amplitudes and phases of the sub-carrier oscillations are thus not corrupted by the adjacent frequencies. The channels are thus orthogonal with respect to one another.
OFDM radio signals can be received and demodulated by means of conventional reception concepts, which are based on the principle of heterodyne reception with subsequent digital quadrature mixing. However, more advanced reception concepts, in which direct-mixing methods are used, are becoming increasingly popular, particularly for reasons relating to lower power consumption and in order to avoid chip-external filters for mirror-image frequency suppression. In the case of direct-mixing receiver concepts (homodyne receivers), the radio signal, which is received via an antenna and is amplified, is split in the front end into an in-phase (I) and a quadrature (Q) path and is mixed directly to baseband in both paths using the output frequency from a local oscillator, with the oscillator frequencies which are supplied to the mixers being shifted through 90° with respect to one another by means of a phase shifter.
However, the occurrence of a direct-current component (DC offset) in direct-mixing receiver structures represents a significant problem, and makes it harder to process the received data, since components can be driven into the saturation range by the DC offset. In the case of orthogonal multicarrier systems, such as OFDM or DMT (Discrete Multi Tone), the addition of a DC offset corresponds in the time domain to the superimposition of an Si function on the spectrum, which corresponds in form and profile to the Si functions of the sub-carriers carrying the data, which maximum occurs at the frequency f=0 Hz. If the sub-carrier in the multicarrier system at the frequency f=0 Hz is not filled with data, this initially has no effects when considered theoretically, since the Si function has zero points at the maxima of all the other sub-carrier frequencies.
However, since a carrier-frequency offset generally also occurs during actual data transmission, this also causes a shift in the transmission spectrum. This means that two effects occur in the event of a DC offset. On the one hand, the maximum of the resultant Si function no longer occurs at the frequency f=0 Hz when the sub-carriers are not filled with data, but at the frequency of the carrier-frequency offset. On the other hand, the shift caused by the carrier-frequency offset results in the zero points of the Si function no longer being located at the sub-carrier frequencies, so that all of the sub-carriers are subject to interference. The individual sub-carriers are in this case interfered with to different extents depending on the profile of the Si function. The greatest interference components occur on the sub-carriers in the vicinity of the maximum of the Si function. In the boundary area, that is to say well away from the maximum of the Si function, the influence of the interference is less, since the amplitude of the superimposed Si function becomes less in these areas. The influence of the DC offset is thus significantly influenced by the amplitude of the DC offset and by the magnitude of the carrier-frequency offset. On the assumption that the amplitude of the DC offset or of the DC interference is constant, it can generally be assumed that the interference will rise as the magnitude of the carrier-frequency offset increases.
Multicarrier systems, in particular OFDM systems, are provided with channel coding for error protection. If a soft input channel decoder is used, the input values to the channel decoder are weighted with reliability information. The value of this reliability information is generally not the same for all of the sub-carriers, and may depend, for example, on the channel states at the sub-carrier frequencies.
German Laid-Open Specification DE 101 14 779 A1 discloses a transmitting and receiving unit which is used in an OFDM multicarrier system. The transmitting and receiving unit is designed such that the interference parameters of an I/Q modulator and of an I/Q demodulator can be detected in an OFDM signal. An OFDM test signal is up-mixed in the transmission path with the carrier frequency from the transmission oscillator to form an RF signal. A frequency interval such as this between the local oscillator and the transmission-end oscillator that corresponds, for example, to a carrier-frequency interval, is chosen in the reception path. The chosen combination of the carrier-frequency interval and of the sub-carriers at the frequency f=0 Hz means that the individual components do not overlap. This means that I/Q components in the transmission path, I/Q components in the reception path, any DC offset in the transmission path and any DC offset in the reception path do not overlap, so that separation is possible in the receiver. It is thus possible to detect and to compensate for the interference components separately from one another with a relatively high degree of complexity, so that DC-compensated signals can be transferred to the channel decoder.